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If you want to add events from an external calendar to one of your calendars in GCal, you can do it only on case-by-case basis. Even a third-party tool won't help you with such a sync because if you import an event from a remote calendar and then delete the event in the remote calendar, it will stay in your GCal (iCal compares content not revisions).

If you just want to display a remote calendar in the same view as your primary calendar, follow these steps:

Click on the arrow to the right of Other calendars in the side bar

Click on Add by URL

Paste the URL and click Add Calendar

Note: Import calendar option in the screenshot is that case-by-case import to add events to your primary calendar.